It looks like a scope problem as "window" is global object and it is not expected to have property "items". Set your firefox to "break on all errors" and when it stops go up the call stack to find the real reason of it.

to get it to work. I think the problem was that since this is a drop target for drag and drop that there where no items in the column model when the grid is first rendered. So calling the init method before I want the form to show makes it get the latest values from the cm.